Referring to FIGS. 1 and 2, typical semiconductor structures utilize a relatively highly doped P.sup.+ 10 (FIG. 1) or N.sup.+ 12 (FIG. 2) substrate having, for example, a dopant concentration generally higher than 10.sup.17 atoms/cubic centimeter. Substrates 10 and 12 are formed using a conventional crystal pulling process so as to have a surface orientation of, for example, &lt;100&gt;, &lt;110&gt;, or &lt;111&gt;.
As shown in FIG. 1, a relatively lightly doped P.sup.- epitaxial layer 13 is formed on substrate 10, and isolation regions 14 of an insulating material are formed from the surface of layer 13 downward into layer 10 to form an isolated device region 15
FIG. 2 shows the same type of isolated device region 22 formed in a lightly doped N.sup.- epitaxial layer 20. Prior to the growth of lightly doped epitaxial layers 13 or 20, a protective capping layer 16 of silicon dioxide is formed on the backside of the substrate to prevent autodoping of the epitaxial layer by the substrate.
Subsequent to the formation of the structures shown in FIGS. 1 and 2, passive devices such as resistors (not shown), and/or active devices (not shown) such as bipolar transistors, field-effect transistors, BICMOS circuits, or integrated circuits are formed in the isolated device regions 15, 22. The formation of these active devices typically includes the formation of multiple doped regions (not shown) within the epitaxial layers 13, 20 of device regions 15 and 22. Many different transistor and device structures, and methods for forming the same, are well known in the art and need not be described here in detail.
The above-described processes and structures suffer from several disadvantages, including: 1) a difficulty in controlling the dopant profiles of the substrates which is well-recognized and endemic in the formation of liquid grown, large crystalline substrates; 2) a similar, endemic inability to control oxygen and other impurities in the substrates; 3) the extra process steps required to form a capping layer such as silicon dioxide or silicon nitride on the substrate back surface; and 4) warping of the substrate caused by such a capping layer.
U.S. Pat. No. 3,585,464 to Castrucci et al. (assigned to the assignee of the present invention) shows a different semiconductor device structure wherein an epitaxial N layer is formed on a P.sup.- substrate. Active devices are subsequently formed in isolated regions of the N layer. The forming of the device-containing layer directly on the lightly doped substrate results in the disadvantage of less than optimal electrical isolation between isolated device regions.
U.S. Pat. No. 3,721,588 to Hays shows a method wherein a lightly doped substrate 10 is used to support a first highly doped epitaxial layer 11. In contrast to the present invention, however, the Hays patent teaches the removal of the substrate and the first epitaxial layer to leave a thin, second epitaxial silicon layer supported on an insulator.